1. Field of the Invention
The present invention relates to a method for making a BaTiO3-based dielectric having a high dielectric constant and a low dielectric loss, and more particularly to a method for making a BaTiO3-based dielectric having a high dielectric constant and a low dielectric loss, in which a BaTiO3-based body is subjected to a pre-heat treatment in a hydrogen (H2) atmosphere or a reducing atmosphere containing mixed gas of hydrogen and nitrogen in a ratio of hydrogen: nitrogen=5 to 100%: 0 to 95% prior to a sintering process in the manufacture of dielectrics, in order to obtain a reduced average grain size of BaTiO3.
2. Description of the Related Art
It has been reported that in a BaTiO3 system, a small number of grains are abruptly grown in an abnormal fashion at a temperature in the vicinity of 1,332xc2x0 C. while consuming grains of a small grain size on the matrix (Y. Matsuo and H. Sasaki, J. Am, Ceram. Soc., 54[9] p471(1971); D. Hennings, Sci. Ceram., 12, p405(1984); D. Kolar, p529 in Ceramic Transaction, Vol. 7, Sintering of Advanced Ceramics. Edited by C. A. Handwerker, J. E. Blendell and W. A. Kaysser. American Ceramic Society, Westerville, Ohio, (1990); P. R. Rios, T. Yamamoto, T. Kondo, and T. Sakuma, Acta Metall. Mater., 46[5] p1617 (1998); H. Schmelz and A. Meyer, Ceram. Forum Int., Ber. Dtsch. Keram. Ges., 59[8/9] p436 (1982); H. Schmelz, Ceram. Forum Int., Ber. Dtsch. Keram. Ges., 61[4/5] p199(1984)).
Such an abnormal grain growth in a BaTiO3 system results in an abrupt increase in the average grain size of a BaTiO3-based sintered body produced using the BaTiO3 system. Due to such an abnormal grain growth in the sintered body, air pores present at grain boundaries in that sintered body are rendered to be combined together, thereby expanding the grains of the sintered body to increase the volume of those grains. As a result, a decrease in sintering density occurs (Marlyse Demartin, Claude Herard, Claude Carry, and Jacques Lemaitre, J. Am. Ceram. Soc., 80 [5] p79 (1977)). For this reason, in order to obtain a BaTiO3-based sintered body having a high relative density and a low average grain size, the above mentioned abnormal grain growth should be suppressed.
It is known that BaTiO3 exhibits superior dielectric characteristics when it has an average grain size of 0.8 to 1.6 xcexcm (K. Kinoshita, and A. Yamaji, J. Appl. Phys., 47(1) p371(1976); G. Arlt, D. Hennings, and G. de with, J. Appl. Phys., 58(4), p1619(1985)).
Various methods have been proposed to make a BaTiO3-based sintered body which has a reduced average grain size of BaTiO3 to exhibit superior dielectric characteristics. In order to reduce the average grain size of the BaTiO3 system, it is necessary to suppress an abnormal grain growth in the BaTiO3 system or to increase the number of nuclei in abnormal grains. The method of suppressing an abnormal grain growth in the BaTiO3 system is typically implemented by the addition of an additive. It has been reported that such an additive may include ZrO2, Dy2O3, or Nb2O5 (T. R. Armstrong, L. E. Morgens, A. K. Maurice, and R. C. Buchanan, J. Am. Ceram. Soc., 72(4) p605(1989); A. Yamaji, Y. Enomoto, K. Kinoshita, and T. Murakani, J. Am, Ceram. Soc., 60(3-4) p108(1977); M. Kahn, J. Am. Ceram. Soc., 54(9) p452(1971)). However, this method cannot be used for BaTiO3 added with an acceptor additive because materials proposed as an additive for suppressing an abnormal grain growth are neutral additives (ZrO2) or donor additives (Dy2O3 and Nb2O5). Although it is necessary to add an additive in an excessive amount for an effective suppression of an abnormal grain growth, such an excessive addition of the additive may cause a variation in the electrical characteristics of pure BaTiO3. For this reason, the above mentioned method has a limited application.
It is also known that it is possible to reduce the average grain size of the sintered body by increasing the number of nuclei in abnormal grains. For the method of increasing the number of nuclei in abnormal grains, there are a fast firing method, in which a sample is sintered at a temperature in the vicinity of 1,460xc2x0 C. higher than a sintering temperature ranging from 1,340xc2x0 C. to 1,380xc2x0 C., and a method in which SiO2 or TiO2 serving to promote a creation of nuclei in abnormal grains is added ((H. Mostaghaci and R. J. Brook, J. Br. Ceram. Soc., 82[5] p167(1983); M. P. Harmer and R. J. Brook, J. Br. Ceram. Soc., 80[5] p147(1981); D. Kolar, p529-45 in Ceramic Transaction, vol 7, Sintering of Advanced Ceramics. Edited by C. A. Handwerker, J. E. Blendell and W. A. Kaysser, American Ceramic Society, Westerville, Ohio, (1990); C. Eastman, C. A. Elyard, and D. Warren, Proc. British Ceram. Soc., 18(1970)). However, the fast firing method is unsuitable for practical applications because it is conducted at a temperature in the vicinity of 1,460xc2x0 C. very higher than a typical sintering temperature ranging from 1,340xc2x0 C. to 1,380xc2x0 C. This method has a drawback in that when SiO2 or TiO2 serving to promote a creation of nuclei in abnormal grains is added, a Ti-rich liquid phase is formed, thereby causing the electrical property of the sintered body to be degraded.
For another grain size adjusting method, there is a hot isostatic pressing method (G. Tomandl, A Stiegelschmitt, and K. Bermuth, CFI-Ceram. Forum Int. -BER. Dtsch. Keram. Ges., 64 [10] p389(1987); K. Oonish, T. Morohashi and K. Uchino, Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi, 97 [4] p473 (1989)). This hot isostatic pressing method is effective in that a reduced sintering temperature can be used. However, it is difficult for this method to be applied to a practical component manufacture because the process used is expensive and complex.
The inventors made active research to solve the above mentioned problems. After such active research, the inventors has developed a process capable of increasing the relative density of a sintered body while reducing the average grain size of the sintered body in accordance with a pre-heat treatment conducted in a reducing atmosphere prior to a sintering process.
Therefore, an object of the invention is to provide a method for making a BaTiO3-based dielectric having a very small average grain size while having a high relative density in accordance with a simple heat treatment conducted for BaTiO3 including pure BaTiO3 or BaTiO3 added with an additive in a reducing atmosphere at a temperature less than a liquid phase forming temperature, prior to a sintering process for sintering the BaTiO3.
In accordance with the present invention, this object is accomplished by providing a method for making a BaTiO3-based dielectric, comprising the steps of: preparing BaTiO3 powder for raw powder; molding the raw powder, thereby producing a powder compact body; subjecting the powder compact body to a pre-heat treatment in a reducing atmosphere; and sintering the powder compact body subjected to the pre-heat treatment.